1. Field of the Invention
The present invention relates to an image sensing apparatus and a method for driving a solid state image sensor. More particularly, the present invention relates to an image sensor that uses a solid state image sensing apparatus employing a frame transfer system and a driving method thereof.
2. Description of the Related Art
An image sensing apparatus, which has a solid state image sensor like a charge coupled device (CCD), controls the exposure of the solid state image sensor to acquire the optimal exposure state. This exposure control employs a iris mechanism, which mechanically controls the amount of incident light to the solid state image sensor in accordance with the luminance of light reflected from a target object to be sensed. Alternatively, the exposure control may use a so-called electronic shutter for controlling the period during which the solid state image sensor accumulates charges in accordance with the luminance of light reflected from the target object.
A solid state image sensor, which accumulates (stores) information charges that are generated in accordance with the incident light, has light-receiving pixels arranged in a matrix form. When an overflow of information charges occurs at some of the light-receiving pixels, excess charges bloom from the light-receiving pixels and the blooming charges tend to get mixed with information charges stored in the light-receiving pixels that are adjacent or located peripherally thereof. To prevent such mixing, an overflow drain is provided adjacent to each light-receiving pixel.
FIG. 1 is a block diagram illustrating a prior art image sensing apparatus. The apparatus has a CCD solid state image sensor employing a frame transfer system. The image sensor 51 has a vertical overflow drain structure to absorb excess information charges on the substrate side. The sensor 51 includes a light-receiving section (image sensing section) 51i, a storage section 51s, a horizontal transfer section 51h and an output section 51d. Each section is formed on a semiconductor substrate (not shown). The light-receiving section 51i is formed by a plurality of shift registers, which are arranged in parallel in a horizontal direction. Each shift register has a plurality of bit registers arranged in series in a vertical direction. The individual bit of those shift registers forms a light-receiving pixel, which stores information charges that are produced in accordance with the light reflected from a target object to be sensed. The storage section 51s is formed by a plurality of shift registers arranged in series to the associated shift registers of the light-receiving section 51i. The number of bits of each shift register in the storage section 51s is set to match the number of bits of each shift register in the light-receiving section 51i. The storage section 51s temporarily stores one screen of information charge packets transferred from the light-receiving section 51i.
The horizontal transfer section 51h is comprised of a single shift register having a plurality of bit registers respectively connected to the outputs of a plurality of the shift registers of the storage section 51s. The horizontal transfer section 51h receives one screen of information charge packets from the storage section 51s row by row and sequentially outputs those information charges to the output section 51d. The output section 51d includes an electrically isolated capacitor, which receives the information charges from the horizontal transfer section 51h pixel by pixel, and an amplifier, which produces a voltage value signal or a image signal Y0(t) in accordance with a potential change in the capacitor.
When there is no incident light reaching the sensor 51, dark current is generated in the semiconductor substrate of the light-receiving section 51i. The dark current gets mixed with the information charges acquired in an image sensing operation and generates noise. The information charges containing the noise are drained out of the light-receiving section 51i by an electronic shutter operation.
Therefore, before the image sensor 51 initiates an image sensing operation, an electronic shutter operation is performed in step (hereinafter referred to as S) 11 in a flowchart of FIG. 2. The operation drains information charges containing noise generated by dark current out of the light-receiving section 51i. In S12, the information charges are stored in the light-receiving section 51i for a predetermined period. In S13, the information charges stored in the light-receiving section 51i are transferred to the storage section 51s. Then, if the apparatus is in the moving image sensing mode, it goes back to S11 and senses the next frame, and if the apparatus is in the still image sensing mode, it finishes the image sensing operation.
However, the prior art image sensing apparatus cannot drain information charges containing noise caused by dark current generated in the substrate out of the storage section 51s.
Generally, in the moving image sensing mode, the storage section 51s stores few information charges containing noise since the image sensor 51 is continually scanned and the information charges are read out therefrom. In other words, dark current generates little noise in the moving image sensing mode.
On the other hand, in the still image sensing mode, the image sensor 51 is not continually scanned and information charges are stored there for a longer period than in the moving image sensing mode. This may increase in the storage section 51s the amount of the information charges containing noise caused by dark current. This increases the noise in the information charges.